A commonly used circuit structure is a "diamond follower". The diamond follower can be used as a differential input stage of a wide-band amplifier. FIG. 1 shows the basic diamond follower circuit. To use the diamond follower as a differential input stage, the collectors of transistors 6 and 8 are connected to complementary inputs of a push-pull output stage. When the diamond follower is used as in a differential input stage, terminal 2 of FIG. 1 is used as the +V.sub.IN terminal and terminal 3 is used as the -V.sub.IN terminal. It is well known that it is very desirable that a differential amplifier have a zero "input offset voltage". That is, when the amplifier output voltage is zero, the voltage difference between the differential input terminals also is zero. However, obtaining a zero input offset voltage for the diamond follower is difficult, because the base-to-emitter voltages (V.sub.BE 's) all need to be equal if the input offset voltage V.sub.OS is to be zero. The transistor V.sub.BE 's are determined by the equation V.sub.BE =V.sub.TH ln(I.sub.C /I.sub.S), where V.sub.TH =(kT/q), and k is Boltzman's constant, T is the junction temperature in degrees Kelvin, q is the electron charge, I.sub.C is collector current, and I.sub.S is saturation current. In a typical integrated circuit process, the NPN transistors such as 6 and 7 are fabricated using different critical processing steps from those used to construct the PNP transistors 5 and 8. In some cases the NPN transistors may be "vertical" devices while the PNP transistors may be "lateral" or "vertical" PNP devices. The "saturation current" of a transistor is a strong function of various processing parameters, including the doping levels on both sides of the emitter-base junction. The emitter-base junctions of the NPN transistors 6 and 7 are formed during entirely different process steps than the process steps forming the emitter-base junction of the PNP transistors 5 and 8. The saturation currents of the NPN transistors 6 and 7 are strongly dependent on the diffusion parameters of the N++ emitter diffusion, and the saturation currents of the PNP transistors 5 and 8 are unrelated to the N++ emitter diffusions. The contact potentials are inherently different for NPN transistors and PNP transistors because the contact potentials are a strong function of the saturation currents. This inherent difference in the contact potentials for NPN and PNP transistors results in inherent, large, somewhat uncontrollable variations in the input offset voltage of the diamond follower circuit of FIG. 1 when it is used as a differential input stage of an amplifier.
A known solution to the problem is to add diode-connected NPN transistors or PNP transistors in series with transistors 5, 6, 7, and 8 so that each NPN transistor emitter-base junction is always connected in series with a corresponding PNP transistor emitter-base junction. These solutions increase the amount of integrated circuit chip area required to implement an amplifier, and have a detrimental effect on circuit performance by adding noise and capacitance that usually results in poor frequency response for the amplifier.
There is a presently unmet need for a simple technique for reducing the input offset voltage of a diamond follower.
There is a presently unmet need for a simple technique for reducing the input offset voltage of an amplifier using a diamond follower as a differential input stage.
A wide variety of current mirror circuits are known. None of them have ever been utilized to compensate for input offset errors in a diamond follower stage used as a differential input stage of an amplifier.